PowerShell Basics: If Statement

PowerShell Basics_ If Statement

Introduction to The Windows PowerShell If Statement

PowerShell’s ‘If’ statement comes under the umbrella of flow control.  Once you master the basic construction then you can increase its usefulness by appending, ‘Else’ and ‘ElseIf’ statements.  One of my favourite alternative strategies is to use ‘Switch‘.

Topics for PowerShell’s If Statement

Construction of the PowerShell ‘If’ Statement

As with so many PowerShell constructions, the type of bracket signifies how to break the script into sections.  It’s worth tattooing into our memory that (parenthesis brackets are for the first part, namely the condition), while {braces are for the block command}.

If (condition) {Do stuff}
# Another explanation would be
If (test) {
"Execute when true"

Summary:  In PowerShell, ‘If’ executes statements conditional on the truth of the test expression.

Example 1a: Plain ‘If’

# PowerShell If Statement Simple Example
$Number = 10
If ($Number -gt 0) {"Bigger than zero"}

Learning Points

Note 1: Trace the above construction, and separate the two components: if (test) and {what to do}.

Note 2: Avoid over-think; there is no ‘Then’ in a PowerShell ‘If’ statement.  My advice is that instead of worrying about ‘If Then’, pay close attention to the two types of bracket.  Furthermore, there is no endif in PowerShell as there is in VBScript.

Note 3: To double check your understanding, try amending, “Bigger than Zero” to a different text string, such as:  “Less than nought”.  Once you have done that, set $Number to -1 in the above Example 1a.

Example 1b: PowerShell Script to Checks If a File Exists

# Windows PowerShell example to check 'If File Exists'
$ChkFile = "C:\Windows\explorer.exe"
$FileExists = Test-Path $ChkFile
If ($FileExists -eq $True) {
Write-Host "Yippee, explorer.exe exists"

Note 4: You probably want to change the file referenced by $ChkFile.  Indeed, the script cries out to be modified for a more constructive {outcome}.

Note 5: You could easily append an else statement to cater for a file not existing.
Else {Write-Host "File does not exist"}

See more on PowerShell’s Test-Path

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If Logic and Tactics

It’s rare that my first ‘If’ construction produces the desired results.  The secret of success is to experiment with the If test, or alternatively start introducing one or more ‘ElseIf tests with their corresponding {outcome block}.

In this example I have introduced -Not into the logic:

Example 1b: PowerShell If -Not Statement to Check for a Service

# PowerShell script to check whether a service is installed
$Name = "Alerter"
$Service = Get-Service -display $Name -ErrorAction SilentlyContinue
If (-Not $Service) {
$Name + " is not installed on this computer."

Note 6: This script uses the Display Name property of the service.  Remember in PowerShell ‘If Then’ does not exist.  Observe it’s plain ‘If’.

Note 7: Observe in passing the If -Not construction; incidentally, -Not is sometimes abbreviated to an exclamation mark ! in PowerShell.

Note 8: This script cries out for an Else statement.  We need to know if the named service has been installed.

# PowerShell If statement to check whether a service is installed
$Name = "Print Spooler"
$Service = Get-Service -display $Name -ErrorAction SilentlyContinue
If (-Not $Service) {$Name + " is not installed on this computer."}
Else {$Name + " is installed."
$Name + "'s status is: " + $service.Status }

Note 9: See more on -ErrorAction SilentlyContinue

Example 1c: PowerShell If Statement To Check IP Addresses

Here is another real-life example of the ‘If statement’ in action.  The idea is to ping 20 IP Addresses.  The logic says If the StatusCode is 0, then display a label saying ‘ON NETWORK’.

# PowerShell If Statement To Test Ip Addresses
$i =1
$Ip = "192.168.1."
Write-Host "IP Address"
Write-Host "----------------------------------------"
Do { $Ip4th = $Ip + $i
$Pingy = Get-WmiObject Win32_PingStatus -f "Address='$Ip4th'"
If($Pingy.StatusCode -eq 0) {
"{0,0} {1,5} {2,5}" -f
$Pingy.Address, $Pingy.StatusCode," ON NETWORK"}
{"{0,0} {1,5} {2,5}" -f $Pingy.Address, $Pingy.StatusCode, " xxxxxxxxx"
until ($i -eq 20)

Note 10: Once If statements get complicated it’s time to investigate PowerShell’s Switch parameter.

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Example 1d: File Content Example of Plain ‘If’

PowerShell has a batch of help files.  One of these files contains help about the ‘if’ statement.   In the example below, $File references that file.  $Content is set to the content of the file.  The third line attempts to match a string to the contents of the file.

# Help on PowerShell's if statements
$File = Get-Help about_if
If ($File -Match "The if Statement") {
"We have the correct help file"

Learning Points

The above example is concerned with matching a string “The if Statement” to the contents of the built in help file.

Example 2a: ‘If’ With ‘Else’

This example deal with plain ‘Else’.  This is a simple command, unlike ElseIf there is no second test construction, ‘Else’ just follows on to reflect what to do if the If statement is false.

# Help on PowerShell's Else statements
$File = Get-Help about_if
If ($File -Match "The if Statement") {"We have the correct help file"}
Else {"The string is wrong"}

Learning Points

The best way to see how ‘else’ operates is to amend line 3 thus:
($File -Match “The ifyyyy Statement”).

Example 2b: Checking If a Service Has Been Installed

# PowerShell script to check whether a service is installed
$Name = "Print Spooler"
$Service = Get-Service -display $Name -ErrorAction SilentlyContinue
If (-Not $Service) {$Name + " is not installed on this computer."}
Else {$Name + " is installed."
$Name + "'s status is: " + $service.Status }

See more on PowerShell Else »

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Example 2c: PowerShell If -Not Logic

The purpose of this script is to employ -Not logic to check for the Alerter service.  One scenario is you are working with Windows 7 machines, and they no longer install this service.

Observe how -Not reverses the ‘If’ logic compared with Example 1.

# PowerShell script to check if a service is -NOT installed
$Name = "Alerter"
$Service = Get-Service -display $Name -ErrorAction SilentlyContinue
If (-Not $Service) {$Name + " is not installed on this computer."}
Else {"You probably have $Name, thus machine is Vista or W2K3"}

Note 11:  See more examples of PowerShell’s If -Not logic.

Example 2d: NetSh

The purpose of this example is to check the status of a firewall on a Windows client machine such as Windows 8.1 or Windows 7.

# PowerShell ElseIf Example Using NetSh
Write-Host "Firewall configuration for $env:computername"
$Fw = NetSh advfirewall set currentprofile state on
If($Fw -Match 'ok'){Write-Host "$env:username's job is done"}
ElseIf($Fw -Match 'requires elevation') {Write-Host "Call for an administrator"}
Else{Write-Host "Nothing happened"}
NetSh advfirewall set currentprofile state on

Researching If, Else and ElseIf

Get-Help About_If

Example 3: ElseIf

This example has a real task; to check that we have the name of an actual file.  Remember that the second (test statement) must be followed by a second {Block Script}.

# Help on PowerShell's ElseIf statements
$Item = "about_if"
$File = Get-Help $Item
If ($File -Match "The if Statement") {"We have the correct help file, $Item"}
 ElseIf ($File.Length -lt 1) {"Check file location"}
Else {"File exists, but does not contain text string"}

See much more on the PowerShell ElseIf construction »

Learning Points

Note 12: The advantage of ElseIf over plain Else, is that we can introduce a new test.  In the above example we use ElseIf to check if the length of the file is less than 1.  To activate the ‘ElseIf’ block, set $File to a non-existent file for example
$File = Get-Help about_ifxx

Note 13: To trigger the final ‘Else’, try changing:
$File = Get-Help about_if
$File = Get-Help about_scope

If you have time, you could add more ‘ElseIf’ statements to cater for other eventualities.  Alternatively, if the ElseIf construction is getting unwieldy, then try the superior switch command.

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If you would another account of PowerShell’s If statement, then have a look at the built-in help file

# For even more information about 'If' and 'ElseIf' constructions:

Get-Help about_If

This is Microsoft’s explanation:

if (<test1>)
{<statement list 1>}
[elseif (<test2>)
{<statement list 2>}]
{<statement list 3>}]

Windows PowerShell evaluates the <test1> conditional expression as either true or false. Should the result be true, PowerShell obeys whatever is inside the {curly brackets}, whereupon PowerShell exits the If statement.

In the event of the first test being false PowerShell works its way through the ElseIf statements.

Incidentally, the ‘Vehicle’ for our tests reveals a whole family of ‘about_zyx…’ files.  My point is there is no command: ‘Get-Help if’, however, there is a help file called, ‘about_if’.  Furthermore, if you look in the PowerShell directory then you will see ‘About’ files to assist with commands such as ‘If’ and ‘ElseIf’.  You can list these ‘About’ files with the command:

See more on Help about*

Summary of PowerShell’s If Construction

When it comes to filtering output, one of the oldest and best statements is the ‘If’ clause.  As usual, the secret of understanding the syntax is to pay close attention to the style of bracket.  If (parenthesis for the test) and {braces for the action}.  Once you have mastered the basic ‘If’ statement, then extend your capabilities by researching ‘Else’ and ‘ElseIf’.

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See more Windows PowerShell flow control examples

PowerShell Home  • PowerShell If Statement  • PowerShell ElseIf   • Free Permissions Analyzer

Conditional Operators  • PowerShell -Match  • PowerShell -Like  • PowerShell -Contains

PowerShell Comparison Operators  • PowerShell Syntax   • Where Filter  • PowerShell Else

Please email me if you have a better example script. Also please report any factual mistakes, grammatical errors or broken links, I will be happy to correct the fault.